32 We will leave it to him to preach two Gods, who can preach One God without confessing the unity; he shall proclaim that God is solitary, who can deny that there are two Persons, Each dwelling in the Other by the power of Their nature and the mystery of birth given and received. And that man may assign a different nature to Each of the Two, who is ignorant that the unity of Father and of Son is a revealed truth. Let the heretics blot out this record of the Son's self-revelation I in the Father and the Father in Me; then, and not till then, shall they assert that there are two Gods, or one God in loneliness.
There is no hint of more natures than one in what we are told of Their possession of the one Divine nature. The truth that God is from God does not multiply God by two; the birth destroys the supposition of a lonely God. And again, because They are interdependent They form an unity; and that They are interdependent is proved by Their being One from One. For the One, in begetting the One, conferred upon Him nothing that was not His own; and the One, in being begotten, received from the One only what belongs to one.
Thus the apostolic faith, in proclaiming the Father, will proclaim Him as One God, and in confessing the Son will confess Him as One God; since one and the same Divine nature exists in Both, and because, the Father being God and the Son being God, and the one name of God expressing the nature of Both, the term 'One God?' signifies the Two. God from God, or God in God, does not mean that there are two Gods, for God abides, One from One, eternally with the one Divine nature and the one Divine name; nor does God dwindle down to a single Person, for One and One can never be in solitude.
Source: On the Trinity (New Advent)